2020A&A...644A..34S

Context. Deuteration has been suggested to be a reliable chemical clock of star-forming regions due to its strong dependence on density and temperature changes during cloud contraction. In particular, the H₃⁺ isotopologues (e.g. ortho-H₂D⁺) seem to act as good proxies of the evolutionary stages of the star formation process. While this has been widely explored in low-mass star-forming regions, in the high-mass counterparts only a few studies have been pursued, and the reliability of deuteration as a chemical clock remains inconclusive.
Aims. We present a large sample of o-H₂D⁺ observations in high-mass star-forming regions and discuss possible empirical correlations with relevant physical quantities to assess its role as a chronometer of star-forming regions through different evolutionary stages.
Methods. APEX observations of the ground-state transition of o-H₂D⁺ were analysed in a large sample of high-mass clumps selected from the ATLASGAL survey at different evolutionary stages. Column densities and beam-averaged abundances of o-H₂D⁺ with respect to H₂, X(o-H₂D⁺), were obtained by modelling the spectra under the assumption of local thermodynamic equilibrium.
Results. We detect 16 sources in o-H₂D⁺ and find clear correlations between X(o-H₂D⁺) and the clump bolometric luminosity and the dust temperature, while only a mild correlation is found with the CO-depletion factor. In addition, we see a clear correlation with the luminosity-to-mass ratio, which is known to trace the evolution of the star formation process. This would indicate that the deuterated forms of H₃⁺ are more abundant in the very early stages of the star formation process and that deuteration is influenced by the time evolution of the clumps. In this respect, our findings would suggest that the X(o-H₂D⁺) abundance is mainly affected by the thermal changes rather than density changes in the gas. We have employed these findings together with observations of H¹³CO⁺, DCO⁺, and C¹⁷O to provide an estimate of the cosmic-ray ionisation rate in a sub-sample of eight clumps based on recent analytical work.
Conclusions. Our study presents the largest sample of o-H₂D⁺ in star-forming regions to date. The results confirm that the deuteration process is strongly affected by temperature and suggests that o-H₂D⁺ can be considered a reliable chemical clock during the star formation processes, as proved by its strong temporal dependence.